This multicenter, randomized, double-blind trial compares the efficacy and safety of 256U87 with acyclovir in the treatment of adult patients who present within 72 hours with a presentation consistent with first episode genital herpes infection. They will be given either 256U87 1 gram orally 2 times a day for 10 days or acyclovir 200 milligrams orally 5 times a day for 10 days and they will be monitored over 2 weeks or until all lesions have healed.